Generally, two major and distinct process steps are employed in the production of high molecular weight polyesters involving solid phase polymerization techniques. The first step involves the preparation of a low molecular weight polyester prepolymer resin using either a batch or a continuous melt phase polymerization process. The resulting molten prepolymer is then solidified and converted into a particulated form such as, for example, pellets, diced cubes, granules, powders, etc., and is known in the art as solid phase feed polymer. This prepolymer or solid phase feed polymer (these terms being used interchangeably throughout this specification) typically will have an intrinsic viscosity in the range from about 0.35 to about 0.60 and a moisture content ranging from about 0.40 to about 0.25 percent by weight and will be amorphous, as evidenced by its having a density of 1.34 grams per cubic centimeter or less.
The second step involves the actual solid phase polymerization of the above-described particulated prepolymer to the desired high molecular weight product. For polyethylene terephthalate, the solid phase polymerization step is normally conducted at temperatures ranging from about 185.degree. C. to about 250.degree. C. and preferably from about 220.degree. C. to about 240.degree. C. in an inert gas atmosphere or in vacuum employing either batch rotary blender-dryers, fluid bed dryers, tray dryers or continuous gravity flow reaction towers.
In addition, it is a generally acknowledged and accepted fact that because of the wet, amorphous nature of this prepolymer it is necessary to subject it to an intermediate step wherein the prepolymer is heated, dried and crystallized to avoid degradation and agglomeration of the prepolymer when it is exposed to solid phase polymerization conditions. It is also known that when subjecting said wet, amorphous prepolymer to this intermediate step the particulated prepolymer will tend to agglomerate into an unmanageable mass, just as in the solid phase polymerization step, unless means are taken to prevent this fusion. One means of preventing fusion of the particulated prepolymer during the heating, drying and crystallization thereof is to subject the prepolymer to vigorous forced motion. Examples of processes for the pretreatment of polyester prepolymer in which vigorous forced motion prepolymer is employed include U.S. Pat. Nos. 4,064,112 and 4,161,578. However, a major drawback to processes employing vigorous forced motion of the particulated prepolymer during the heating, drying and crystallization thereof is the generation of polymeric fines which can account for from about 0.06 to about 0.11 percent by weight of the total weight of prepolymer being treated. The presence of these fines in the prepolymer is undesirable not only because they give rise to handling problems but also because of the adverse effect they have on the quality and uniformity of the final polymeric product and its performance when subjected to operations such as melt spinning and injection molding.
Another approach to the heating, drying and crystallization of particulated wet amorphous polyester prepolymer prior to the solid phase polymerization thereof is the process described in U.S. Pat. No. 3,634,359. In this patent is disclosed an improvement in a process for the preparation of high molecular weight polyesters, particularly high molecular weight polyethylene terephthalate, by after-condensing the corresponding low molecular weight polyester in the solid phase. The specific improvement in the process is stated to be the drying, crystallizing and heating of the low molecular weight prepolymer to solid phase polymerization temperatures by means of high frequency energy in the presence of a streaming dry gas.
In carrying out the process described in the above-referenced patent it is stated that the use of a streaming dry gas and the manner in which the heating with high frequency is performed are extremely important limitations on the successful operation of the process. According to this patent, the use of a streaming dry gas is essential for the instant withdrawal of water set free from the polyester during the heating process if hydrolytic decomposition of the polyester is to be maintained within tolerable limits and that the heating must be conducted in such a manner that the range of temperature in which crystallization takes place with optimum speed is not exceeded before the polyester has adapted the crystallite form in order to avoid melting the granulated polyester into a solid lump. The specific manner in which the heating is to be conducted is disclosed as consisting of either slowly heating the polyester to condensation temperature, i.e. between 5 and 15 minutes with constant movement of the granulated polyester or rapidly heating the polyester to 100.degree. C. to 180.degree. C., maintaining the polyester in a field-free space at the temperature reached until it is sufficiently crystallized and then and only then subjecting said crystallized polyester to further high frequency heating to raise the temperature thereof to condensation temperatures. The deleterious effects which occur as a result of not using a streaming dry gas and heating too rapidly are illustrated in Example 3 of the patent. In that example granulated polyethylene terephthalate was heated to 240.degree. C. within 5 minutes and in the absence of a streaming dry gas. The effects of such a rapid heating and lack of a streaming dry gas were that the material melted together into a solid clump and that the specific viscosity (which is a measure of its molecular weight) of the material was reduced from an original value of 0.88 to a value of 0.75, a 17 percent decrease.